Transmissions

ABSTRACT

A transmission ratio selecting mechanism including a ratio selector means (typically a hand lever), first and second ratio coupling members (36b,37 ) movable to select a number of ratios in the forward and reverse drive directions of an associated transmission, and linkage means (65,63,54,61,62,59,57,60,65,63,64,70,71,68,67,69) for moving the ratio coupling members in response to movement of the ratio selector means. The ratio selecting mechanism includes disconnecting means (72,73,63,64) for disconnecting one (37) of the ratio coupling members from the selector means during selection of the reverse ratios of the transmission. During selection of any reverse ratio both the gear coupling means are initially moved in synchronism in response to movement of the selector means to complete the first part of the selection of the particular reverse ratio, the disconnecting means is then operated, and selection of the reverse ratio is completed by further movement of only one (36b) of the ratio coupling means in response to further movement of the selector means. The selector means is arranged to return the ratio coupling members to synchronism on movement of the selector means to disengage the reverse ratio.

TECHNICAL FIELD

This invention relates to transmissions and in particular totransmission ratio selecting mechanism.

In such fields as, for example, tractors and forklift trucks there is arequirement for transmissions which provide several ratios in theforward and reverse drive directions and a manual forward/reverseshuttle facility.

The applicant's UK Pat. No. 2,055,163 (Based on U.K. Patent ApplicationNo. 7,925,589) describes and claims an improved form of transmisionassembly which provides two forward and two reverse drive ratios andwhich when used in combination with one or more other transmission unitsis capable of providing a large number of forward and reverse driveratios.

The provision of an efficient and compact ratio selecting mechanism fora transmission assembly of the type described and claimed in theapplicant's above referred to patent application and other transmissionassemblies in which the selection of ratios involves the movement ofmore than one ratio coupling member (typically an axially movable sleeveor collar) is a difficult problem.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improved form ofratio selecting mechanism which is compact and which is suitable for usewith transmission assemblies of, for example, the type described andclaimed in the applicant's previously referred to patent application.

According to the present invention there is provided a transmissionratio selecting mechanism including a ratio selector means, first andsecond ratio coupling members movable to select a number of ratios of anassociated transmission, and linkage means for moving the ratio couplingmembers in response to movement of the ratio selector means, the ratioselecting mechanism being characterised in that the linkage meansincludes disconnecting means for disconnecting one of the ratio couplingmembers from the selector means during selection of at least one of thetransmission ratios, the arrangement being such that for the selectionof said at least one ratio both the gear coupling means are initiallymoved in synchronism in response to movement of the selector means tocomplete the first part of the selection of said at least one ratio, thedisconnecting means is then operated, and selection of said at least oneratio is completed by further movement of only one of the ratio couplingmeans in response to further movement of the selector means, theselector means being arranged to return the ratio coupling members tosynchronism on movement of the selector means to disengage said at leastone ratio.

The linkage means may include a member which is angularly movable inresponse to movements of the selector means with first and second cammembers arranged to be moved by the angularly movable member to move thefirst and second ratio coupling members respectively, and with thedisconnecting means arranged to disconnect drive from the angularlymovable member to one of the cam members during selection of said atleast one ratio.

Conveniently the disconnecting means includes a toggle linkage connectedwith the selector means for displacing the angularly movable member in adirection generally parallel to its axis of angular movement todisconnect drive from said one cam member.

In one form of selecting mechanism in accordance with the invention, inwhich the coupling members are required to be moved in oppositedirection to select said at least one ratio, the selector means may bearranged on initial movement to move both coupling members in the samedirection to couple a gear wheel associated with one of said couplingmembers with an associated shaft and on further movement to operate saiddisconnecting means to move only the other coupling member in theopposite direction to couple a gear associated with the other couplingmember with a further shaft to complete the selection of said at leastone ratio.

A selecting mechanism of the form described in the preceding paragraphis particularly suitable for use in selecting the ratios of thetransmission assembly of the applicant's previously referred to patentapplication.

The first ratio engaging member comprises a coupling sleeve forming partof a non-synchromesh clutch unit and the second ratio engaging membermay comprise a coupling sleeve forming part of a synchromesh clutch unitof the type specified, the linkage means ensuring that thenon-synchromesh coupling sleeve is brought into engagement first duringthe selection of each ratio of the associated transmission.

References throughout this specification to synchromesh units or clutchunits of the type specified refer to units which include a couplingmember (for example a sleeve) axially slidable relative to a shaft forcoupling a gear to said shaft, the coupling member being slidable from anonengaged position in which said gear is not coupled to the shaft,through a synchronising position in which clutch members in thesynchromesh unit are in frictional contact to synchronise the rotationalspeeds of said gear and shaft to a fully engaged position in which saidgear is coupled to said shaft and axial movement of the coupling memberis completed.

In a further form of the invention the linkage means includes aselecting mechanism characterised in that the linkage means includes twogenerally parallel shift rails one for each ratio coupling member, and aselector member connected with the ratio selector means, the ratioselector member having two formations engageable with cooperatingformations associated with the two rails, the formations on the selectormember being movable in directions generally parallel to the directionof extension of the rails to move said rails and also being movableduring the selection of said at least one ratio in directions generallyperpendicularly to said directions of extension in order to disconnectone of the formations on the selector means from its cooperating railformation thus disconnecting the associated ratio coupling member fromthe ratio selector means.

Where the selecting mechanism is used in the control of synchromeshclutch unit of the type specified using cam member as described abovethe cam member associated with the synchromesh sleeve preferably hasthree zones, a first zone contacted by the follower during movement ofthe coupling member towards the synchronising position, a second zonecontacted by the follower during synchronisation and a third zonecontacted by the follower after synchronisation has taken place and asthe coupling member is moved towards the fully engaged position, thefirst, second and third zones being shaped so that a tangent to the camsurface at the point of contact with the follower during synchronisationmakes a smaller angle with the direction of movement of the cam memberthan tangents drawn to points of contact with the first and third zonesof the cam surface so that for a given selection force applied to thecam member a larger ratio engagement is applied to the follower duringsynchronisation than when the follower is in contact with the first andthird zones.

DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a vertical section through an agricultural tractortransmission embodying the invention;

FIG. 2 is a diagrammatic representation of part of an input gear set Zof the transmission part of which is not visible in FIG. 1;

FIG. 3 shows the provision of an additional planetary gear to double thenumber of ratios provided by the transission.

FIG. 4 shows the speed selection gate of the transmission;

FIG. 5 shows the sequence of movement of the synchronising devices of amain change speed gear set X of the transmission;

FIG. 6 shows the cam groove used to control the synchronising devices ofthe main change speed gear set X;

FIG. 7 shows diagrammatically the manner in which the cam having thegroove of FIG. 6 is rotated;

FIG. 8 shows in plan the manner in which the input set cam and auxiliaryoutput set cam are sequenced relative to each other;

FIG. 9 is a sectional view on the line A--A of FIG. 8;

FIG. 10 shows the cam grooves which control the auxiliary output set Yand the non-synchromesh coupler of the input gear set Z;

FIG. 1 shows a side view in the direction of arrow B of FIG. 8 of theupper half of the cam arrangement shows in FIG. 8;

FIG. 12 shows the range selection gate of the transmission;

FIG. 13 shows the cam groove which controls the synchromesh coupler ofthe input gear set Z;

FIG. 14 shows the sequence of movements of the synchromesh coupler ofthe input gear set;

FIG. 15 shows graphically the ground speeds obtainable (inKilometers/hour) in each ratio of a typical transmission in accordancewith the invention at selected engine speeds.

FIG. 16 shows the sequence of movements of the couplers of the auxiliaryoutput gear set Y and the non-synchromesh coupler of the input gear setZ;

FIGS. 17 and 18 show in tabular form the gear wheels used to obtain eachtransmission ratio;

FIG. 19 shows part of the cam groove of FIG. 13 on an enlarged scale;

FIG. 20 is a simplified representation of the geometry of the camfollower and groove shown in FIG. 19;

FIG. 21 shows a vertical section through an alternative ratio selectionarrangement for input gear set Z and main change speed gear set X, and

FIG. 22 shows schematically the plate-type cam member used in thearrangement of FIG. 21.

BEST MODE OF CARRYING OUT INVENTION

The basic transmission construction is shown in FIG. 1. It comprises amain four speed change speed gear set X, a two speed auxiliary outputgear set Y, and a constant mesh input gear set Z capable of providingtwo forward speeds and two reverse speeds.

The main change speed set X comprises four gears 10, 11, 12 and 13 whichare splined onto a lay shaft 14 and four constant mesh gears 15, 16, 17and 18 which are rotatably mounted on the two portions 19 and 20 of thetransmission output shaft.

The gears 15 and 16 are associated with clutch means in the form of afirst synchronising unit 21 while the gears 17 and 18 are associatedwith further clutch means in the form of a second synchronising unit 22.

As will be appreciated, a first ratio is provided in the main changespeed set X by sliding the sleeve 21a of the synchronising unit 21 tothe left so that power is transmitted from the shaft 19 through gears 15and 10 onto the lay shaft 14 and back to portion 20 of the output shaftvia the auxiliary output train Y. In a similar fashion a second ratio isobtained by sliding the sleeve 21a to the right thus transmitting powerfrom the shaft portion 19 via gears 16 and 11 to the lay shaft 14 andhence to the output shaft portion 20 via the output set Y.

Similarly, two further ratios can be obtained by sliding the secondsleeve 22a of the synchronising unit 22 to the left and rightrespectively.

The output gear set Y comprises a gear 23 slidable on splines 24provided on the output shaft portion 20. This gear 23 is engageable witha gear 25 which rotates with the lay shaft 14 to provide a low auxiliaryoutput train ratio. By sliding the gear 23 on the splines 24 teeth 26 onthe gear 23 can be meshed with co-operating teeth 27 on the gear 18 thuslocking the gear 18 to the shaft portion 20 and providing a higheroutput train ratio via gears 13 and 18.

The input gear set Z comprises gears 28 and 29 which are rotatablymounted on an input shaft 30. These two gears are in constant mesh withgears 31 and 32 which are in turn rotatably mounted on the output shaftportion 19. Mounted behind the gears 28, 29, 21 and 32 as viewed in FIG.1, are two idlers gears 33 and 34 shown diagrammatically in FIG. 2 whichmesh with gears 32 and 28 respectively. The idler gears rotate with acommon shaft 35.

Associated with the gears 28 and 29 is a further clutch means in theform of a third synchronising unit 36 whose sleeve 36b can be used tocouple either the gear 28 or the gear 29 to the input shaft 30. Theclutch means for coupling either the gear 31 or 32 to the output shaft19 comprises a simple sliding collar 37 which is coupled with shaft 19and engageable with teeth 31a and 32a provided on gears 31 and 32respectively.

The input gear set Z can be arranged to provide a high forward ratio bymoving the sleeve 36b to the left to couple the gear 28 to the inputshaft 30 and also sliding the collar 37 to the left to contact the gear31 with the shaft portion 19. A low forward ratio can be provided byengaging the sleeve 36b with the gear 29 and sliding the collar 37 tothe right to couple the gear 32 with the shaft portion 19.

A first reverse ratio is obtained from the input set Z when the sleeve36b couples the gear 28 to the input shaft 30 and the collar 37 couplesthe gear 32 to the shaft portion 19. With the synchronising sleeve andthe collar in these positions drive passes from the input shaft 30 viagear 28, gear 34, shaft 35, gear 33 and gear 32 to the shaft portion 19.A further reverse ratio is obtained by engaging the synchronising sleeve36b with the gear 29 and moving the collar 37 to couple the gear 31 withthe shaft portion 19. In this condition drive is transmitted from theinput shaft 30 via gear 29, gear 32, gear 33, shaft 35, gear 34, gear 28and gear 31 to the shaft portion 19.

Thus the input set Z, is capable of providing two forward speeds and tworeverse speeds which enable the whole transmission to provide, ifrequired, sixteen forward speeds and sixteen reverse speeds.

Input set Z is the subject of the applicants previously referred to U.K.Pat. No. 2,055,163 (Based on U.K. Patent Application No. 7,925,589).

Since, as described below, the collar 37 is always arranged to coupleeither the gear 31 or 32 to the output shaft portion 19 before thesynchronising sleeve 36b couples either the gear 28 or 29 to the inputshaft 30, the collar 37, when coupling the gear 31 or 32 to the shaftportion 19, only has to overcome the relatively low rotational inertiaof the gears of input train Z since the higher inertia items such as theshaft 30 and the associated clutch parts (not shown) are not at thattime coupled by the synchronising device 36. This makes it possible touse a simple sliding collar 37 rather than another synchronising unitthus making input set Z significantly cheaper and also more compact thanif another synchronising unit was used in place of collar 37. In thislater regard it will be noted that the maximum radial dimension ofcollar 37 is significantly less than that of sleeve 36b thus allowingshafts 30 and 19 to be appreciably closer together than if collar 37were to be replaced by another synchronising unit. This allows the wholetransmission to be of a more compact design.

If desired, the speed range can be further increased by the addition ofa two speed planetary gear on the input end of the transmission as shownin FIG. 3. The planetary gear includes a sun wheel 38 formed integrallyon an input shaft extension 39, planet gears 40 mounted on a planetcarrier 41, and an annulus gear 42 which is held fixed.

A slidable sleeve 43 can occupy the position shown in FIG. 3 in which itcouples the planet carrier 41 with the previously described input shaft30 to provide a first planetary ratio or can be slid axially to engageteeth 44 formed on the input shaft extension 39 to directly couple theinput shaft extension to the shaft 30 thus locking out the planetarygear to provide a second ratio.

It will be appreciated that mechanical coupling of the two speedplanetary gear using the sleeve 43 can be replaced by a hydraulicallyoperated clutch arrangement.

The two speed planetary gear, whether mechanically or hydraulicallycoupled, can be used as an under-drive or overdrive as required.

As indicated above the transmission (without the use of the two speedplanetary gear shown in FIG. 3) provides sixteen forward speed andsixteen reverse speeds.

These forward speeds are grouped into four ranges A, B, C and D as shownin FIG. 15. Range A is a creeper range, ranges B and C are the fieldworking ranges and range D is the road speed range. FIG. 15 indicatesthe ground speeds obtainable (in Kilometer/hour) in each ratio of atypical transmission embodying the invention at selected engine speedsand clearly illustrates the flexible nature of the transmission and thegood spread of ground speeds. The legend at the top right hand corner ofFIG. 15 indicates the selected engine speed p, q, r and s on the blockmarking, these speeds being 2400, 2040, 1730 and 1400 engine revolutionsper minute respectively (2400 revs/min being the rated engine speed ofthe engine).

Selection of these ranges is under the control of a lever (not shown)which moves in a gate 66 shown in FIG. 12. Each range A, B, C and D hasfour sub-ratios 1 to 4 selected by a second lever (not shown) whichmoves in a straight gate 49 of FIG. 4.

Also, the gate 66 controls the selection of the four basic reverseranges R1, R2, R3 and R4 which again each have four sub-ratios 1 to 4controlled by gate 49.

Thus using the two gates 49 and 66 sixteen forward and reverse ratioscan be selected.

The transmission also provides a "park" facility which is selected ongate 49 and designated "P" and which locks the transmission up tofunction as a parking brake.

All the ratios of the main change speed set X, the two speed auxiliaryoutput set Y, and the constant mesh input set Z are selected byrotatable cams having cam tracks which are engaged by followers which inturn move gear selector forks to make the required gear changes.

The main change speed set X is controlled by the lever which moves downthe straight gate 49 shown in FIG. 4. As can be seen between each ratioposition a neutral condition is provided and the park facilitydesignated "P" is available in which the set X is locked-up bysimultaneously engaging the two synchronising devices 21 and 22.

The left and right hand portions of FIG. 5 show schematically theshifting sequence of the two synchronising units 21 and 22 respectively.FIG. 5 indicates the synchronising units are arranged so that normallywhen one synchronising unit is operated to engage a gear the othersynchronising unit is in its neutral condition N. The facility isprovided to simultaneously engage the first and fourth gear ratiosprovided by gears 15 and 18 in order to place the transmission in itspark condition P.

The schematic shift pattern shown in FIG. 5 is in practice translatedinto a single cam groove 51 formed in the periphery of a rotatable cammember 50. For ease of reference the development of this cam member isshown in FIG. 6 from which it will be readily apparent that the lengthEF of the groove 51 corresponds to the left hand portion of FIG. 5 andthe lenght GH of the groove 51 corresponds to the right hand portion ofFIG. 5. The two cam followers which operate in groove 51 thus share acommon neutral portion GI of the groove. This arrangement in which twofollowers operate in the same groove 51 greatly simplifies the camarrangement and saves a considerable amount of space inside thetransmission housing by avoiding the need for two separate cams andtheir associated rotating means.

The cam 50 is shown diagrammatically in FIG. 7 and is arranged to berotated by a toothed sector 52 which engages teeth 53 formed on the cam.Thus pivoting of the toothed sector 52 as indicated by the arrow W inFIG. 7 results in rotation of the cam 50 as indicated by the arrow K.

FIG. 8 shows the cam operating mechanism used in the selection of theratios in the auxiliary output set Y and the input set Z. The cam 50 ispositioned behind this cam arrangement as viewed in FIG. 8, the sector52 being secured to a sleeve 54 which is rotated by a lever 55 coupledwith the gear selection lever which moves in gate 49.

As can be seen from FIG. 8 the lever 55, sleeve 54, sector 52 and cam 50are carried by a detachable cover 120 which is located on the right handside of the transmission housing in the position indicated by dottedcircle 120 in FIG. 1. This cover has support webs 121 and 122 whichproject through a circular aperture 123 in the housing and which supportthe cam 50 and cams 59 and 68 referred to below.

Turning now to the control of the auxiliary output set Y and themovement of the collar 37, this is under the control of cam tracks 56and 57 cut in the periphery of a rotatable cam 59. These cams, whosedevelopments are shown in FIG. 10, are representations of the shiftpattern for the auxiliary output set Y and the collar 37 as shownschematically in FIG. 16. Thus the cam groove 56 corresponds to the lefthand portion of FIG. 16 and the groove 57 corresponds to the right handportion of FIG. 16.

A cam follower shown at 60 in FIG. 11 engages the groove 57 while acorresponding follower (not shown) engages the groove 56. As thesefollowers follow along their respective grooves the necessary axialsliding movement of the associated selector forks to effect the movementof the gear 23 and the collar 37 occurs. As will be appreciated, sincethe throw required to change the operating ratio of the auxiliary outputset Y is large this must be reflected in the sideways throw T of the camgroove 56 which is appreciably greater than that of groove 57.

As can be seen from FIGS. 10 and 11 by appropriate operation of the cam59 four different speed ratios can be obtained using the two positionsof the gear 23 and the two alternative positions of the collar 37.

The cam 57 is rotated by a geared sector 61 which engages a gear 62formed on the cam 59 in a similar manner to the way in which theselector 52 engages the gear 53. The sector 61 is splined onto a shaft63 at 64 and the shaft 63 is rotated via an arm 65 connected with thegear lever (not shown) which moves in the gate 66 shown in FIG. 12.Shaft 63 is again carried by cover 120. Thus movement of the lever whichmoves in the gate 66 rotates the shaft 63 and hence rotates the cam 59via sector 61 and gear 62.

Control of the synchronising unit 36 is effected by a cam groove 67 cutin a cam 68. The development of this groove is shown in FIG. 13 and isagain simply the shift pattern for the synchronising unit 36 as shownschematically in FIG. 14.

The cam groove 67 is engaged by the follower 69 shown in FIG. 11.Rotation of the cam 68 is effected by a geared sector 70 which issplined onto the shaft 63 at 64 and drives a gear 71 formed on the cam68. As previously described, for forward drive ratios the synchronisingsleeve 36b and collar 37 are either both moved to the left or to theright from their neutral positions. This requires synchronisation of therotation of cams 59 and 68. This synchronisation is achieved by virtueof the fact that both gear sectors 61 and 70 are rotated from the samesplined connection 64 on the shaft 63.

When the input train Z is operated to give a reverse drive ratio thesynchronising sleeve 36b and collar 37 are moved one to the left and oneto the right of their neutral positions in order to transmit drive tothe idler gears 33 and 34.

In order that the synchronising sleeve 36b and collar 37 can occupypositions on opposite sides of their neutral position it is necessary tobreak the synchronisation of the rotation of cams 59 and 68. This isachieved by axially displacing the shaft 63 so that the splines 64disengage the gear sector 61 whilst remaining engaged with the sector70. This sliding of the shaft 63 is achieved by either pulling orpushing on the cable 72 which operates the toggle linkage 73 secured tothe end of the shaft 63.

Thus, for example, when reverse range R1 is selected on the gate 66 thisis achieved in two states. Firstly, if the lever is not already in therange A position it is moved to this position and in so doing rotatesthe two cams 59 and 68 in synchronism under the control of shaft 63 tothe appropriate position for range A. Next the lever is moved from therange A position to the reverse range R1 position. This movement firstlydisplaces the cable 72 which in turn moves the shaft 63 thus disengagingsector 61 and hence leaving the output set Y and collar 37 in the rangeA position and secondly rotates the cam 68 in the reverse direction tomove the synchronising sleeve 36b from the range A position to the rangeR1 position.

As will be appreciated the transmission provides four manualforward/reverse shuttle changes between the A, R1: B, R3 and C, R4positions. Each of these shuttle changes involves the movement of onlythe synchronising coupling sleeve 36b to the right or left as the casemay be.

FIGS. 17 and 18 show in tabular form the gear wheels used in thetransmission shown in the drawings to provide each of the transmission'sthirty-two ratios. It has been found possible to arrange for several ofthe gear wheels used in the transmission to have the same number ofteeth enabling either actual common gear wheels or at least common gearwheel hobbing machinery to be used to provide these gear wheels thusgiving a significant economy in the manufacture of the transmission bystill further reducing the number of different gear wheels provided. Inone example of the transmission described, gear wheel 11 has the samenumber of teeth as gear wheel 28, gear wheel 13 has the same number ofteeth as gear wheel 28, gear wheel 16 has the same number of teeth asgear wheel 29, and gear wheel 18 has the same number of teeth as gearwheel 31.

As stated above ranges B and C are the two field working ranges whilerange D is the road speed range. Thus, for example, when a tractor isbeing operated in a field with a particular implement it is normallyoperating in either range B or range C and there is normally littlerequirement to switch between these two ranges although the operatorwill require the facility to switch from which ever of these ranges isbeing used to the road speed D range on, for example, his journey to andfrom the field. For this reason the range selection gate 66 shown inFIG. 12 is laid-out with the road speed range D positioned between thetwo field working ranges B and C so that range D can be reached directlyfrom either range B or range C. It will also be seen from aconsideration of FIG. 17 that changing to range D from range B or rangeC requires only one change in the transmission coupling. For example,changing to range C from range D only involves the sliding of couplingsleeve 36b and changing to range B from range D only involves thesliding of gear 23. This compares favourably with changes between rangesB and C which involve sliding of both coupling sleeve 36b and gear 23.Thus, with the range D position between the range B and C positions,changes between each of the adjacent ranges on the gate 66 only involvesone change in the transmission coupling.

The profiles of cam grooves 56 and 57 are basically straight or largeradius curved tracks jointed together smoothly by the use of appropriateradii 56a, 56b and 57a, 57b (see FIG. 10). This simple profile ispossible since the ratio changes controlled by these grooves are notsynchronised.

However, the grooves 51 and 67 which control the synchronised changesare "wavy" in nature. Thus for example, referring to FIG. 13 the groove67 makes the change from portion 67a to 67b by way of radiused portions67c, 67d, 67e and 67f on the left side of the groove and complementaryradiused portion 67g, 67h, 67i, and 67j on the other side of the groove.The wavy shape of these grooves is the subject of the applicantsco-pending patent application No. EP80/00053 and now abandoned (Based onU.K. patent application No. 7,925,588).

Comparison of FIGS. 10 and 13, which are laid out on the same scale,indicates that after a given cam movement R of cam 68 a sideshift S awayfrom neutral of the follower engaged in the groove 67 has occurredwhereas for the same movement R of cam 59 a larger sideshift S1 isobtained. This ensures that the sleeve 37 is engaged with teeth 31a or32a before the synchronising unit 36 couples the gear 28 or 29 to theshaft 30.

In practice the sideshift S is arranged to take up the clearances in thesynchronising unit and bring the synchronising clutch parts 36a intoengagement while the sideshift S1 is sufficient to partially engage thesleeve 37 with either teeth 31a or 32a. After synchronisation has takenplace the groove 67 effects further axial sliding of the synchronisingcoupling sleeve 36b thus coupling gear 28 or 29 to the shaft 30. Duringthis further movement along groove 67 the sleeve 37 is moved to its fullengaged position by groove 57 completing a total sideshift movement ofS2 as shown in FIG. 10.

The shape of groove 67 and the manner in which it effects movement ofthe associated follower and sleeve will now be discussed in more detail.FIG. 19 shows part of the groove 67 of FIG. 13 on an enlarged scale.Dotted circle 100 indicates the neutral position of the cam followerwhich engages the groove 67 at contact point L. If a force F is nowapplied to the cam 68 this will cause the cam follower to move thedistance S to the right as viewed in FIG. 19, as a result of contactwith the zone P of the side of the groove 67, to take up the clearancesin the synchronising clutch parts into frictional engagement in order tocarry out their synchronising action. Dotted circle 101 shows theposition of the follower when the synchronising clutch parts areactually carrying out the synchronising action and the follower iscontacting the groove 67 at the point M on zone Q of the cam.

As can be seen from FIG. 19 tangents 102 and 103 to the side of thegroove 67 at contact points L and M respectively are inclined at anglesα' and α respectively to the line of action of the force F applied tothe cam from the selector lever. Typically angle α' will be 45 degreesand angle α will be less than 20 degrees and typically 8 to 13 degrees(say 10 degrees). The use of a small angle α during synchronisationensures that the axial force FF actually applied to the follower andhence to the associated selector fork is as large as possible so thatthe effort which must be applied to the gear selector lever in order toengage any synchromesh gear is maintained as low as possible.

The fact that the force FF is larger for smaller angles of α can bereadily seen from the simplified two dimensional representation of thegeometry of cam follower and groove as shown in FIG. 19 in which the camgroove wall is represented by the straight line β at an angle α to thedirection of application of the force F applied to the cam.

If the cam follower initially contacts the wall (line β) at point O andthe F force moves the cam through a small peripheral distance OO' sothat the wall occupies the dotted line position β' it will be seen thatthe point of contact of the follower with the groove wall will havemoved axially to the position O".

Equating the work input and output of the cam and roller system we have:##EQU1##

Thus as α decreases so that value of FF increases for a given value ofF.

After synchronisation has taken place the synchronising coupler is freeto move axially and engage the desired gear ratio, circle 104 in FIG. 19showing the fully engaged position. This movement of the synchronisingcoupler is brought about by the contact of the zone N of the side of thegroove 67 (corresponding to say portion 67d or FIG. 13) with the groovefollower. Since it is not necessary to apply the same level of force tothe synchronising coupler after synchronisation has occurred the curvedzone N of the side of the groove is generally inclined at a largerangle. This larger inclination also means that a larger axial movementof the selector fork and associated coupler can be obtained for a givenamount of rotation of the cam when the follower is operating on the zoneN of the side of the groove.

Thus, the shape of the zones P and N of the side of the groove 67 arechosen to give a large axial movement of the cam follower for a givenamount of rotation of the cam while the shape of zone Q adjacent thecontact point M is chosen to provide the greatest axial force duringactual synchronisation.

It will be appreciated that at the contact point

    L FF.sub.L =F/tan 45°=F/1

Similarly at the contact point M

    FF.sub.M =F/tan 10°=F/0.1736

Thus the force generated at the follower is approximately 5.7 timesgreater at contact point M. Since the force required duringsynchronisation is approximately five times more than required at theneutral L position this results in a substantially constant load beingrequired at the selector lever for all selector lever positions.

Typically the mean force multiplication ratio of the selector mechanism##EQU2## is about 5 to 1 while this ratio ratio is about 2.3 to 1 at theneutral position and about 13 to 1 when synchronisation is occurring.The average multiplication ratio of the cam on its whole travel is##EQU3## Typically this ratio is of the order of 2 to 1.

The above described "wavy" cam shape is also employed in the other partsof cam 68 and on cam 50 which controls synchronising units 21 and 22.

Also, although in the construction described above, the cam tracksengaged by the various followers are provided by grooves cut into cammembers it will be understood that the tracks could be provided byraised circumferentially extending projections formed on the peripheralsurfaces of the cams. Additionally instead of each follower movingbetween opposed walls provided by a groove or raised projections, thefollower could be biased into engagement with a single circumferentiallyextending wall.

FIG. 21 shows an alternative form of ratio selection arrangement forinput set Z and main change speed gear set X.

The input set Z is controlled by a selector member 130 which ispivotally supported at 131 on a rotatable sleeve 132 carried by aremovable cover 133 which closes a transmission casing aperture similarto the aperture 123 referred to above. The lower end of member 130 isconnected at 134 with a twin pronged selector member 135 which issplined at 136 for axial sliding movement within sleeve 132.

The prongs 137 and 138 of selector member 135 engage recesses 139 and140 in members 141 and 142 carried by selector rails 143 and 144respectively. Rails 143 and 144 are connected with synchronising unit 36and collar 37 respectively.

Selector member 130 is connected with a selector lever 145 as showndiagrammatically at 146 in FIG. 24, this lever being movable along anS-shaped gate shown diagrammatically at 147.

FIG. 21 shows the lever in a neutral position in which neithersynchronising unit 36 nor collar 37 is coupling any of the gears 28, 29,31, 32 or input set Z to their associated shafts. In order to couplegears 29 and 32 with shafts 30 and 19 the selector member 130 is rotatedwith the sleeve 132 and member 135 about axis 148 from the positionshown in FIG. 21 in a counter clockwise sense when viewed in thedirection of arrow C by the movement of lever 145 to the position A/C ofgate 147. This moves both selector rails 143 and 144 rearwardly thuscoupling gears 29 and 32 to shafts 30 and 19. As indicated by the gatemarking A/C either range A or range C is engaged by this movement oflever 145 depending on the position of gear 23 of set Y. In thisalternative selection arrangement the movement of gear 23 can beachieved by a convenient means, for example, this can be achievedelectro-hydraulically at the press of a button which operates asolenoid-operate valve controlling the supply of hydraulic fluid to ahydraulic actuator.

In order to ensure that collar 37 engages before synchronising unit 36the clearances in the connecting linkage (not shown) between prong 137and the synchronising unit 36 are arranged to be greater than theclearances in the connecting linkage (not shown) between prong 138 andthe collar 37.

In a similar fashion movement of lever 145 to the B/D position rotatesthe selector member 135 in a clockwise sense about axis 148 thuscoupling gears 28 and 31 to shafts 30 and 19 and engaging range B or Ddepending on the position of gear 23.

A reverse RA is obtained by pivotting lever 145 to the left from the B/Dposition to pivot member 130 in an anticlockwise sense about pivot 131and thus disengaging prong 138 from recess 140. This disconnects therail 144 from the selector mechanism and allows the selection of reverserange RA to be completed by the movement of the lever 145 to the RAposition of gate 147 which rotates member 135 in a counter clockwisesense about axis 148 and moves sleeve 36b rearwardly to couple gear 29to shaft 30.

A further reverse range RB is obtained by pivotting lever 145 to theright from the A/C position to pivot member 130 in a clockwise senseabout pivot 131 and thus disengage prong 138 from recess 140 into recess139 with prong 137. This disconnects the rail 144 from the selectormechanism and allows the selection of reverse range RB to be completedby the movement of the lever 145 to the RB position of gate 147 whichrotates member 135 in a clockwise sense about axis 148 and moves sleeve36b forwardly to couple gear 28 to shaft 30.

As will be appreciated two reverse ranges are selectable with the gearlever in the RA position dependent on the position of gear 23 andsimilarly two reverse ranges are selectable with the gear lever in theRB position dependent on the position of gear 23.

The four speed main change speed gear set X is conrolled by a rotaryselector member 150 which is supported by the cover 133 and is connected(as shown diagrammatically at 151) with a further selector lever 152which moves in a straight gate 153. Member 150 is connected by shaft 154with a plate-type cam member 155 (in a manner not shown) so thatrotation of member 150 rotates shaft 154 and thus cam 155 about an axis156.

Cam member 155 is shown schematically in FIG. 22 when viewed in thedirection of arrow C and includes a slot 157 for the passage of selectormember 135 and two cam tracks 158 and 159 for the operation ofsynchronising units 22 and 21 respectively. Each cam track is engaged bya separate follower 160, 161, only the follower 161 of cam track 158being visible in FIG. 21, and the centres of both followers 160, 161 arelocated and maintained on line 170 in FIG. 22.

With the lever 152 in the position shown in FIG. 21 both followers linein the neutral positions indicated by dotted circles 160 and 161 in FIG.21. Each follower is connected with the associated synchronising unit byan arm and shift rod as shown by arm 162 and rod 163 of follower 161.

As will be appreciated if the lever 152 is rotated rearwardly to thesecond speed position the cam member 155 is rotated counter clockwise asviewed in FIG. 22 and the followers occupy the position 160' and 161'respectively thus moving the sleeve 21a rearwardly to engage the secondspeed of the main change speed gear set X.

In a similar fashion if lever 152 is rotated forwardly from the neutralposition shown in FIG. 21 to the third speed position cam member 155 isrotated in a clockwise sense as viewed in FIG. 22 and the followersoccupy the positions 160" and 161" in the cam tracks thus moving thesleeve 22a forwardly to engage the third speed of the gear set X.

As will be appreciated the first and fourth speeds of gear set X can beselected by movement of the lever 152 to the appropriate positions ofgate 153 and results in follower positions 160"', 161"' and 160"" and161"" respectively in FIG. 22.

As indicated above FIG. 22 shows the cam member 155 schematically andthe two cam tracks 158 and 159 are not depicted as having the `wavy` camshape described above in detail with reference to FIGS. 19 and 20. Itwill be understood, however, that in practice the wavy cam shape will beused on cam tracks 158 and 159 to reduce the lever effort required atsynchronisation.

I claim:
 1. A transmission ratio selecting mechanism including a ratio selector means, first and second ratio coupling members (36b, 37) movable to couple associated gear wheels (28, 29:31, 32) with first (30) and second (19) shafts respectively to select a number of ratios of an associated transmission, and linkage means (65, 63, 54, 61, 62, 59, 57, 60:65, 63, 64, 70, 71, 68, 67, 69) for moving the ratio coupling members in response to movement of the ratio selector means, the ratio selecting mechanism being characterised in that the linkage means includes disconnecting means (72, 73, 63, 64) for disconnecting one (37) of the ratio coupling members from the selector means during selection of at least one of the transmission ratios, the selecting mechanism being arranged so that for the selection of said at least one ratio both the ratio coupling members (36b, 37) are initially moved axially of their respective shafts (30, 19) in synchronism in the same direction by the selector means so that one coupling member (37) couples one of the associated gear wheels (31, 32) with one (19) of the shafts thus completing the first part of the selection of said at least one ratio, the disconnecting means is then operated, and selection of said at least one ratio is completed by further movement of only the other (36b) coupling member axially of its respective shaft (30) in the opposite direction in response to further movement of the selector means to couple another of the associated gear wheels (28, 29) with the other shaft (30), the selector means being arranged to return the ratio coupling members to synchronism on movement of the selector means to select another ratio.
 2. A selecting mechanism according to claim 1 characterised in that the linkage means includes a member (63) which is angularly movable in response to movements of the selector means, first (68) and second (59) cam members being arranged to be moved by the angularly movable member to move said first (36b) and second (37) ratio coupling members respectively, and the disconnecting means (72, 73, 63, 64) is arranged to disconnect drive from the angularly movable member to one (59) of the cam members during selection of said at least one ratio.
 3. A selecting mechanism according to claim 2 characterised in that the disconnecting means (72, 73, 63, 64) is arranged to displace the angularly movable member (63) in a direction generally parallel to its axis of angular movement to disconnect drive to said one cam member (59).
 4. A selecting mechanism according to claim 3 characterised in that the disconnecting means includes a toggle linkage (72, 73) connected with the selector means for displacing the angularly movable member (63).
 5. A selecting mechanism according to claim 3 characterised in that the angularly movable member comprises a shaft (63) with a splined portion (64) which engages first and second drive members (61, 70) for angularly displacing the first and second cam (68, 59) respectively, the disconnecting means (72, 73) being arranged to axially displace the splined shaft (63) to disengage one (61) of the drive members from the splined portion (64).
 6. A selecting mechanism according to claim 5 characterised in that the drive members each comprise a toothed sector (61, 70) which engages the splined portion (64) and gear teeth (62, 71) associated with each respective cam (59, 68)
 7. A selecting mechanism according to claim 2 characterised in that the cam members (59, 68) each comprise a cylindrical member with a cam formation (57, 67) formed around its periphery, each formation being engaged by a cam follower (60, 69) arranged to move the associated coupling member (36b, 37).
 8. A selecting mechanism according to claim 1 characterised in that the linkage means includes two generally parallel shift rails (143, 144) one for each ratio coupling member (36b, 37) and a selector member (135) connected with the ratio selector means (145), the ratio selector member having two formations (137, 138) engageable with co-operating formations (139, 140) associated with the two rails (143, 144), the formations on the selector member being movable in directions generally parallel to directions of extension of the rails to move said rails and also being movable during the selection of said at least one ratio in directions generally perpendicularly to said directions of extension in order to disconnect one (138) of the formations on the selector means from its co-operating rail formation (140) thus disconnecting the associated ratio coupling member (37) from the ratio selector means (145).
 9. A selecting mechanism according to claim 2 characterised in that the first ratio coupling member comprises a coupling sleeve (37) forming part of a non-synchromesh clutch unit and the second ratio coupling member comprises a coupling sleeve (36b) forming part of a synchromesh clutch unit of the type specified, the linkage means ensuring that the non-synchromesh coupling sleeve (37) is brought into engagement first during the selection of each ratio of the associated transmission.
 10. A selecting mechanism according to claim 2 characterised in that the first ratio coupling member comprises a coupling sleeve (37) forming part of a non-synchromesh clutch unit, the second ratio coupling member comprises a coupling sleeve (36b) forming part of a synchromesh clutch unit of the type specified, the first and second cam members (68,59) being arranged to be rotated at the same speed when moving in synchronism and the relative configurations of said first and second cam members ensuring engagement of the non-synchromesh coupling sleeve (37) first during the selection of each ratio of the associated transmission.
 11. A selecting mechanism according to claim 2 characterised in that the first ratio coupling member comprises a coupling sleeve (37) forming part of a non-synchromesh clutch unit, the second ratio coupling member comprises a coupling sleeve (36b) forming part of a synchromesh clutch unit of the type specified, and the cam member (68) associated with the synchromesh sleeve (36b) has a cam formation (67) for engagement by a follower (69) connected with the synchromesh sleeve, the cam formation having three zones, a first zone (P) contacted by the follower during movement of the coupling member towards the synchronising position, a second zone (Q) contacted by the follower during synchronisation and a third zone (N) contacted by the follower after synchronisation has taken place and as the coupling member is moved towards the fully engaged position, the first, second and third zones being shaped so that a tangent (103) to the cam surface at the point of contact (M) with the follower during synchronisation makes a smaller angle (α) with the direction of movement (F) of the cam member than tangents (102) drawn to points of contact (L) with the first and third zones of the cam surface so that for a given selection force applied to the cam member a larger ratio engagement (FF) is applied to the follower during synchronisation than when the follower is in contact with the first and third zones. 